The invention concerns a spray nozzle for a powder duster for dusting freshly printed products. The spray nozzle is provided with a connection for a tube for supplying a powder-air mixture, a distribution chamber and at least two spray channels which fan-off from the distribution chamber. The invention also concerns a method for dusting freshly printed products.
As is known in the art, freshly printed products, e.g. printed sheets with still wet printing ink, cannot be stacked since the ink would smear. For this reason, these printed sheets are dusted with powder, with the powder particles forming spacers between the printed sheets to prevent direct contact between the sheets and permit drying of the ink. During dusting of the printed sheets, one must ensure that, if possible, the entire powder is disposed on the surface of the printed sheet with as little powder as possible reaching the surroundings. For this reason, dusting apparatus are generally provided with suction systems which vacuum-off the powder which does not adhere to the sheet. The currently used spray nozzles have a spray angle of approximately 90°, and up to twelve spray nozzles of this type are mounted to a spray bar of approximately 1 m in length for dusting the printed sheet as evenly as possible using these spray nozzles. It has turned out that the printed sheet is dusted but a relatively large amount of powder reaches the surroundings. If the spray pressure of the powder-air mixture is increased, the powder rebounds from the surface of the printed sheet and even more powder is discharged into the surroundings. For this reason, the spray pressure of the powder-air mixture cannot be increased. Since the powder-air mixture is discharged with a relatively low pressure, the slightest air currents disturb the powder-air flow.
Ionisation systems are sometimes provided for electrostatically neutralizing the printed sheet to improve adhesion of the powder to the printed sheet. Moreover, in these dusting systems, an electrostatic field is present around the spray nozzles, thereby completely neutralizing the powder and the supporting air. This ensures that the printed sheet and the powder do not repel each other, since both are electrostatically neutral. As a result thereof, the powder distribution on the sheet is improved and less powder is consumed, since less powder migrates into the surroundings. However, the associated expense is relatively large due to installation of an ionisation system.
It is the underlying purpose of the invention to provide a spray nozzle which reduces the powder consumption in an inexpensive manner.